How to update a value in a json file and save it through node.js

How do I update a value in a json file and save it through node.js?
I have the file content:

var file_content = fs.readFileSync(filename);
var content = JSON.parse(file_content);
var val1 = content.val1;

Now I want to change the value of `val1` and save it to the file.

//change the value in the in-memory object
content.val1 = 42;
//Serialize as JSON and Write it to a file
fs.writeFileSync(filename, JSON.stringify(content));

// read file and make object
let content = JSON.parse(fs.readFileSync('file.json', 'utf8'));
// edit or add property
content.expiry_date = 999999999999;
//write file
fs.writeFileSync('file.json', JSON.stringify(content));

For those looking to add an item to a json collection

function save(item, path = './collection.json'){
if (!fs.existsSync(path)) {
fs.writeFile(path, JSON.stringify([item]));
} else {
var data = fs.readFileSync(path, 'utf8');
var list = (data.length) ? JSON.parse(data): [];
if (list instanceof Array) list.push(item)
else list = [item]
fs.writeFileSync(path, JSON.stringify(list));
}
}

I would strongly recommend not to use synchronous (blocking) functions, as [they hold other concurrent operations][1]. Instead, use asynchronous [fs.promises][2]:

```js
const fs = require('fs').promises

const setValue = (fn, value) =>
fs.readFile(fn)
.then(body => JSON.parse(body))
.then(json => {
// manipulate your data here
json.value = value
return json
})
.then(json => JSON.stringify(json))
.then(body => fs.writeFile(fn, body))
.catch(error => console.warn(error))
```

Remeber that `setValue` returns a pending promise, you'll need to use [.then function][3] or, within async functions, the [await operator][4].

```js
// await operator
await setValue('temp.json', 1) // save "value": 1
await setValue('temp.json', 2) // then "value": 2
await setValue('temp.json', 3) // then "value": 3

// then-sequence
setValue('temp.json', 1) // save "value": 1
.then(() => setValue('temp.json', 2)) // then save "value": 2
.then(() => setValue('temp.json', 3)) // then save "value": 3
```


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Save data after task completion
```
fs.readFile("./sample.json", 'utf8', function readFileCallback(err, data) {
if (err) {
console.log(err);
} else {
fs.writeFile("./sample.json", JSON.stringify(result), 'utf8', err => {
if (err) throw err;
console.log('File has been saved!');
});
}
});
```

Doing this asynchronously is quite easy. It's particularly useful if you're concerned with blocking the thread (likely). Otherwise, I'd suggest Peter Lyon's answer

```ts
const fs = require('fs');
const fileName = './file.json';
const file = require(fileName);

file.key = "new value";

fs.writeFile(fileName, JSON.stringify(file), function writeJSON(err) {
if (err) return console.log(err);
console.log(JSON.stringify(file));
console.log('writing to ' + fileName);
});
```

The caveat is that json is written to the file on one line and not prettified. ex:

```json
{
"key": "value"
}
```

will be...

```json
{"key": "value"}
```
To avoid this, simply add these two extra arguments to `JSON.stringify`

```ts
JSON.stringify(file, null, 2)
```

`null` - represents the replacer function. (in this case we don't want to alter the process)

`2` - represents the spaces to indent.

Promise based solution [Javascript **(ES6)** + Node.js **(V10 or above)**]


const fsPromises = require('fs').promises;
fsPromises.readFile('myFile.json', 'utf8')
.then(data => {
let json = JSON.parse(data);
//// Here - update your json as per your requirement ////

fsPromises.writeFile('myFile.json', JSON.stringify(json))
.then( () => { console.log('Update Success'); })
.catch(err => { console.log("Update Failed: " + err);});
})
.catch(err => { console.log("Read Error: " +err);});

If your project supports Javascript **ES8** then you could use **asyn/await** instead of native promise.

addition to the previous answer add file path directory for the write operation

fs.writeFile(path.join(__dirname,jsonPath), JSON.stringify(newFileData), function (err) {})